Sir Mordred is a monster encountered on the top floor of Keep LeFaye during the Merlin's Crystal quest. He must be defeated to force Morgan le faye to reveal the method needed in order to free Merlin. When you land the finishing blow, Morgan le faye will appear and ask you to spare his life. You may choose to kill him, which will cause the message "you kill Mordred". You may also choose to spare his life which will simply result in Morgan le faye revealing the information you need. Nothing drops regardless of the option you pick.